Parkinson's disease (PD) is an adult-onset neurodegenerative disease of the substantia nigra characterized by resting tremor, bradykinesia, muscular rigidity and postural instability. The worldwide prevalence of PD is estimated at 100-200 cases per 100,000 and represents the second most common cause of neurodegenerative disease. Although most PD cases are considered sporadic, approximately 10-20% are familial in nature. Unfortunately, it is estimated that only ~30-40% of the underlying genetic causes of familial PD have been explained to date. The identification of causative mutations for Mendelian disorders is critical for our understanding of their pathogenesis. The combination of linkage analysis with exome sequencing techniques has proven to be an effective strategy to rapidly identify novel causative genes in affected families. However, this methodology can be difficult to apply to late-onset diseases such as PD and amyotrophic lateral sclerosis (ALS) due to the intrinsic difficulties of finding large informative multi-generational pedigrees. Recently, we have developed a methodology to identify causal mutations by using exome sequencing results of index familial cases (i.e. one affected member per family) in an unbiased genome-wide rare variant analysis and successfully identified a novel causative gene for familial ALS. Here we propose to apply this approach towards the identification of novel causal genes for familial PD. The Specific Aims of this proposal are: (1.) Discovery of Novel Candidate Causal Familial PD Genes. Approximately 1,200 index familial PD samples collected by the Investigators and Coriell Institute will be exome sequenced. The exomes will be compared to over 9,000 control exomes in an unbiased genome-wide rare variant analysis to identify novel candidate causal genes. (2.) Validation/Replication of Causal Familial PD Genes. Top candidate genes/variants will be further evaluated by testing for segregation in affected family members and genotyping a secondary panel of controls. Variants that do not segregate properly or are present at a signification percentage in controls will be excluded and the causative nature of the corresponding gene will be re-evaluated. Remaining genes will be sequenced in a replication panel of 405 familial and 302 sporadic PD cases to evaluate the prevalence of mutations in these genes. (3.) Functional Analysis of Causative Familial PD Mutations. The functional consequences of identified mutations will be evaluated though the use of several model systems. The pathogenic effect of the mutations in induced pluripotent stem cells, Drosophila and yeast models will be studied using established assays. We are confident that the proposed project will lead to the discovery of one or more novel causative genes for PD. The identification of additional causative genes will lead to an increased knowledge of the defects contributing to this devastating disease and open new avenues of research for the PD scientific community as well as the development of new therapeutic targets.